Buggy code:
protected static ImageIcon _addAssocIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Association" ) ; protected static ImageIcon _packageIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Package" ) ; protected static ImageIcon _modelIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Model" ) ; protected static ImageIcon _innerClassIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "InnerClass" ) ; protected static ImageIcon _nodeIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Node" ) ; protected static ImageIcon _componentIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Component" ) ; protected static ImageIcon _dataTypeIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "DataType" ) ; protected static ImageIcon _actorIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Actor" ) ; protected static ImageIcon _useCaseIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "UseCase" ) ; protected static ImageIcon _extendIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Extend" ) ; protected static ImageIcon _extensionPointIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "ExtensionPoint" ) ; protected static ImageIcon _includeIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Include" ) ; protected static ImageIcon _dependencyIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Dependency" ) ; protected static ImageIcon _permissionIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Permission" ) ; protected static ImageIcon _usageIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Usage" ) ; protected static ImageIcon _parameterIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Parameter" ) ; protected static ImageIcon _operationIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Operation" ) ; protected static ImageIcon _signalIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "SignalSending" ) ; protected static ImageIcon _stereotypeIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Stereotype" ) ; protected static ImageIcon _guardIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Guard" ) ; protected static ImageIcon _transitionIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Transition" ) ; protected static ImageIcon _classifierRoleIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "ClassifierRole" ) ; protected static ImageIcon _associationRoleIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "AssociationRole" ) ; protected static ImageIcon _callActionIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "CallAction" ) ; protected static ImageIcon _interactionIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Interaction" ) ; protected static ImageIcon _actionIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "CallAction" ) ; protected static ImageIcon _receptionIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Reception" ) ; protected static ImageIcon _commentIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Note" ) ; protected static ImageIcon _messageIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Message" ) ; protected static ImageIcon _flowIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "Flow" ) ; protected static ImageIcon _stateMachineIcon = ResourceLoaderWrapper . getResourceLoaderWrapper ( ) . lookupIconResource ( "StateMachine" ) ; protected JScrollPane namespaceScroll ; protected JComboBox namespaceComboBox ; protected JTextField nameField ; protected JComboBox stereotypeBox ; protected JScrollPane supplierDependencyScroll ; protected JScrollPane clientDependencyScroll ; protected JScrollPane targetFlowScroll ; protected JScrollPane sourceFlowScroll ; protected JScrollPane constraintScroll ; protected JPanel namespaceVisibilityPanel ; protected JCheckBox specializationCheckBox ; protected JScrollPane elementResidenceScroll ; private UMLModelElementNamespaceComboBoxModel namespaceComboBoxModel = new UMLModelElementNamespaceComboBoxModel ( ) ; private UMLModelElementStereotypeComboBoxModel stereotypeComboBoxModel = new UMLModelElementStereotypeComboBoxModel ( ) ; public PropPanelModelElement ( String name , int columns ) { } public PropPanelModelElement ( String name , ImageIcon icon , Orientation orientation ) { } public PropPanelModelElement ( String name , ImageIcon icon , int columns ) { } public PropPanelModelElement ( ) { } public void navigateUp ( ) { } public void navigateNamespace ( ) { } public PropPanel getPropertyPanel ( ) { } public boolean isModuleEnabled ( ) { } public Vector getModulePopUpActions ( Vector v , Object o ) { } public boolean shutdownModule ( ) { } public boolean initializeModule ( ) { } public void setModuleEnabled ( boolean enabled ) { } public boolean inContext ( Object [ ] o ) { } private void initialize ( ) { nameField = new org . argouml . uml . ui . UMLTextField2 ( new UMLModelElementNameDocument ( ) ) ; stereotypeBox = new org . argouml . uml . ui . UMLComboBox2 ( stereotypeComboBoxModel , ActionSetModelElementStereotype . SINGLETON ) ; <START_BUG> namespaceComboBox = new org . argouml . uml . ui . UMLComboBox2 ( namespaceComboBoxModel , ActionSetModelElementNamespace . SINGLETON ) ; <END_BUG> JList namespaceList = new org . argouml . uml . ui . UMLLinkedList ( new UMLModelElementNamespaceListModel ( ) ) ; namespaceList . setVisibleRowCount ( 1 ) ; namespaceScroll = new JScrollPane ( namespaceList ) ; JList supplierDependencyList = new org . argouml . uml . ui . UMLLinkedList ( new UMLModelElementSupplierDependencyListModel ( ) ) ; supplierDependencyScroll = new JScrollPane ( supplierDependencyList ) ; JList clientDependencyList = new org . argouml . uml . ui . UMLLinkedList ( new UMLModelElementClientDependencyListModel ( ) ) ; clientDependencyScroll = new JScrollPane ( clientDependencyList ) ; JList sourceFlowList = new org . argouml . uml . ui . UMLLinkedList ( new UMLModelElementSourceFlowListModel ( ) ) ; sourceFlowScroll = new JScrollPane ( sourceFlowList ) ; JList targetFlowList = new org . argouml . uml . ui . UMLLinkedList ( new UMLModelElementTargetFlowListModel ( ) ) ; targetFlowScroll = new JScrollPane ( targetFlowList ) ; JList constraintList = new org . argouml . uml . ui . UMLMutableLinkedList ( new UMLModelElementConstraintListModel ( ) , null , ActionNewModelElementConstraint . SINGLETON ) ; constraintScroll = new JScrollPane ( constraintList ) ; namespaceVisibilityPanel = new org . argouml . uml . ui . UMLButtonPanel ( new UMLElementOwnershipVisibilityButtonGroup ( this ) ) ; specializationCheckBox = new UMLElementOwnershipSpecificationCheckBox ( this ) ; JList elementResidenceList = new org . argouml . uml . ui . UMLLinkedList ( new UMLModelElementElementResidenceListModel ( ) ) ; elementResidenceScroll = new JScrollPane ( elementResidenceList ) ; } }

Fix:
namespaceComboBox = new org . argouml . uml . ui . UMLComboBox2 ( namespaceComboBoxModel , ActionSetModelElementNamespace . SINGLETON , true ) ;